Sonic, or acoustic, tools are used in the oil and gas industry to measure the travel time of a compressional wave as it travels through a subterranean formation. Acoustic logging tools may also measure shear and velocity waves which may be useful in understanding the rock mechanical properties. Acoustic logs may also be used to determine porosity, lithography, and types of fluid in the bore hole, and to evaluate geo-mechanical subterranean rock properties. For LWD and wireline acoustic logging tools, it is generally desirable to produce acoustic dipole waves emitting from the tool at a broad range of frequencies, including low frequencies down to 250 Hz or even lower. The acoustic dipole waves travel through a wellbore and the subterranean formation and may be observed at an array of receivers arranged on the tool, typically about three to eight feet from the acoustic transmitter.